Why wash plastic parts?

Another newbie question here. Sorry!

Why wash plastic parts before assembling them? Like for putting togeher plastic buildings for your layout. What is on the plastic that should be washed off before putting the parts together? Does it help the paint to adhere to the parts better?

There is usually a small amount of mold release agent ( which makes it easier to get the parts out of the molds) left on the parts. It would interfere with paint adhesion and so needs to be washed off before painting. It’s frequently easier to do this before assembly. I also wa***he assembled model again before painting to remove dust and sanding particles.
Have Fun,
Tom Watkins

I wahe parts in a sink of warm water with Dawn dish soap. Is this the correct way to wahe plastic parts?

Make sure you rince well and then get the parts dry to stop paint flowing where it should not.

I was sceptical about the need to wash parts at first, but after a really bad experience using acrylic model paint on a plastic structure, I now wash and prime all my models prior to painting. I keep an old tooth bru***o make sure the models are clean in amongst all the detial parts.

you might even want to use distilled water
Dave Nelson

Yep, I thoroughly rinse and dry the pieces before I paint them.

and the worst thing is finger prints and skin oils…use washed gloves to handle the parts during and after rinsing…Nitrile gloves are good for this. Latex are not as good.